Fostex 750 User manual

Brand: Fostex

Model: 750

Category: Musical Equipment

Type: User manual

50165-597, Issue 3

Model 750 Audio Mixer

User Guide

Issue 3, May 1997

This User Guide is applicable for serial numbers 201 and later.

5520 West Touhy Avenue

Skokie, Illinois 60077 U.S.A.

Telephone (847) 676-9177

Fax (847) 982-0747

www.studio-tech.com

Model 750

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 3

Model 750

Table of Contents

Foreword

Introduction .................................................................. 7

System Features.......................................................... 8

Installation .................................................................... 10

Configuration ............................................................... 16

Operation ..................................................................... 23

Technical Notes ........................................................... 36

Specifications............................................................... 38

Appendix A-1 ............................................................... 40

Appendix A-2 ............................................................... 41

Block Diagrams

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 5

Model 750

Foreword

With all the pressures in life to get things done faster, work more hours, finish one job so

you can get on to the next, its rare to get a chance to try and do something really right.

Thats what Studio Technologies attempted to do with the Model 750 Audio Mixer. The

goal was simple, to make a great audio mixer for ENG, SNG, and mobile broadcast appli-

cations. By great we meant not just meeting specifications or price issues, but to address

the real needs of personnel out in the field. And not only to meet their immediate needs,

but to provide resources that they can adopt and use in the future. While getting some-

thing perfect is impossible, I can say that I'm very pleased with how the product turned

out, and hope that you share my enthusiasm. I think that you'll find the Model 750 a plea-

sure to use, providing the resources required for efficient, easy-to-use, day-after-day

operation.

A special thanks to Tony Raven, Chief Engineer at Shook Electronics USA. It was Tony

who suggested that Studio Technologies create an audio mixer for mobile broadcast

applications. He supported his idea by spending many hours with me on the telephone,

reviewing lists of features, making comments, and keeping us focused on meeting the

needs of field personnel. Thanks again Tony!

Many other broadcast professionals contributed their time, expertise, and good ideas to

help make the Model 750 what it is. Without their help the unit would definitely not be as

finely tuned as it is. Thanks to you all!

Many people at Studio Technologies contributed toward making the Model 750 a reality.

Fred Roeck did the mechanical design and prepared the production documentation.

Carrie Loving designed the graphics and provided engineering CAD support. Al Lux de-

signed the printed circuit boards. Mitch Budniak, Ace Consulting Engineer, designed the

power supply and did several overall reality reviews. Jim Cunningham, with assistance

from Gary Hebert at THAT Corporation, designed the limiter/compressor circuit. Joe

Urbanczyk designed the computerized test routines for the Audio Precision System One

and worked on agency approvals. Larry Leviton wrote the software that drives the itty-bitty

micro-controller in the Setup section800 bytes in C! Minas Serbekian and his crew built

the prototype units, ensuring that the final design was "manufacturable."

Please contact me with your questions, comments, and suggestions. I can be reached by

voice at (847) 676-9177, fax at (847) 982-0747, or via the Internet @ www.studio-tech.com.

Sincerely,

Gordon Kapes

President

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 7

Model 750

Introduction

The unique requirements of electronic

news-gathering (ENG) trucks, satellite

news-gathering (SNG) trucks, and special-

ized remote broadcast applications

demand more than just a general-purpose

audio mixer. Thats why Studio Technolo-

gies created the Model 750, a special

piece of equipment for a special applica-

tion. Only by taking the time to speak with

industry professionalsdesigners of ENG

and SNG vehicles, along with numerous

field technical personnelwere the neces-

sary specifications established. Using that

information as a road map to guide the

process, additional features, operating

ergonomics, and overall look-and-feel

Figure 1. View of Model 750 Front Panel

Figure 2. View of Model 750 Back Panel

issues were addressed. The result is a

high-performance, easy-to-use audio

mixer expressly designed to go on-the-

road.

What This User Guide Covers

This User Guide is designed to assist you

when installing, configuring, and using the

Model 750 Audio Mixer. A limited amount

of background technical information is

also provided. Should you require detailed

technical information please refer to the

Model 750 Service Guide. The Service

Guide contains detailed service informa-

tion, including schematic diagrams. The

Service Guide is not shipped with each

Model 750, but is available from the fac-

tory upon request.

Issue 3, May 1997 Model 750 User Guide

Page 8 Studio Technologies, Inc.

Model 750

System Features

Mic/Line Inputs

Four transformer-coupled input channels

are provided for connection to micro-

phone or line-level signals. The circuitry is

ruggedized for reliability under tough

operating conditions. A full set of features

is provided for each input channel, includ-

ing selectable input sensitivity, +48V

phantom power, low-cut filter, phase

reverse, level control, and pan pot. For

operator assistance, a bi-color LED pro-

vides signal present and peak level indica-

tion. Two illuminated push-button switches

control the routing of the input signal to

the main and auxiliary (AUX) audio buses.

For convenience the four mic/line input

sensitivity buttons, like all the Model 750s

operator controls, are located on the front

panel.

Stereo Line Inputs

Two stereo line-level input channels are

provided, and are intended for connection

with audio signals associated with video

playback. The channels support real

world broadcast applications, where left

and right signals may, or may not, be an

actual stereo pair. Routing buttons allow

each left and right input to be assigned to

the left output bus, to the right output bus,

to both output buses, or muted. A level

control, along with a bi-color signal

present/peak LED, supports each left

and right input. Two illuminated buttons

control the routing of the signals to the

main and AUX audio buses.

Main Output

Signals from the mic/line input channels,

stereo line input channels, and the setup

section (discussed later) combine to

create the main audio output. The main

output bus is stereo, but can be used to

create two independent monaural audio

mixes. Two studio-quality, stereo limiter/

compressor circuits are provided to con-

trol the dynamic range of the main audio

bus. Far from a simple clipper, each

limiter/compressor circuit utilizes a sophis-

ticated laser-trimmed voltage-controlled

amplifier (VCA) integrated circuit for quiet,

low-distortion operation. For installation

flexibility, three sets of stereo output cir-

cuitry provide access to the main stereo

bus: two are electronically balanced, line-

level; the third is transformer-balanced,

with its output level selectable for mic or

line.

AUX Output

The Model 750 provides a second stereo

audio bus that can be configured to oper-

ate in either of two modes: as an auxiliary

(AUX) output or as a monitor interrupt

function. In the AUX output mode special

applications are easily supported, such as

creating a mix-minus feed or serving as

a second stereo output for on-air use.

Signals from the mic/line input channels,

stereo line input channels, and the setup

section can be assigned to the AUX bus.

Access to the AUX bus is provided by

means of an electronically-balanced, line-

level output.

In most cases, however, the AUX interrupt

mode will be selected, allowing the normal

monitor source to be interrupted and the

AUX signal monitored in its place. Model

750 operators will find this function ex-

tremely useful, enabling one or more of

the input channels, or the setup section,

to be monitored without interfering with

signals assigned to the main output bus.

Two DIP switches, accessible from the

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 9

Model 750

front panel, allow the interrupt function to

be configured; one switch selects whether

the monitor speaker output will be inter-

rupted by the AUX signal, the other

whether the headphone output will be

interrupted.

Configurable Outputs

Four monaural, balanced, line-level out-

puts can serve in a wide range of installa-

tion-specific applications. DIP switches,

located on the back panel, are used to

select from the five available signal

sources: main output left, main output

right, AUX output left, AUX output right,

and mic/line input channel direct. The DIP

switches can be set to provide additional

stereo outputs, create monaural outputs

from the main or AUX outputs, or to pro-

vide direct outputs from the mic/line input

channels.

Monitor Section

Reminiscent of much larger audio con-

soles, the Model 750s monitor section

gives the operator an extensive set of

resources. Separate stereo outputs and

level controls are provided for connection

to a monitor amplifier and headphones. In

addition to monitoring the main and AUX

audio buses, provision has been made for

monitoring two external audio sources.

These stereo inputs are intended to be

connected to off-air, microwave, or satel-

lite receivers. External Monitor Input 1 is

compatible with +4 signals, while Exter-

nal Monitor Input 2 can be set, using a

front-panel DIP switch, for 10 or +4

compatibility.

Multiple sources can be simultaneously

selected for monitoring. This can prove

useful by allowing, for example, an opera-

tor to simultaneously monitor a local mix

and an externally-provided mix-minus

signal. A mono function allows the moni-

tored audio source to be checked for

phase cancellation problems and accept-

able sum-to-monaural performance.

The AUX interrupt mode allows monitoring

of selected input channels and the setup

section without interfering with normal

operation of the main audio bus. For

operator assistance, the meters can be

configured to monitor the AUX bus when-

ever the AUX interrupt mode is active.

Mechanical VU-type meters, along with

peak level LED indicators, provide a user-

friendly indication of audio-signal levels.

Fanatics for addressing the details, Studio

Technologies has even eliminated the

issue of burned-out meter-scale illumina-

tion by implementing a solid-state, LED-

based lighting scheme.

Setup Section

Unique to the Model 750 are the features

provided by the setup section, the high-

light being the ability of the operator to

record and playback three voice identifica-

tion segments. The Left+Right segment

allows a voice message of up to 12 sec-

onds in length to be recorded, with play-

back going to the left and right output

channels. The Left Only and Right Only

segments allow recording of up to 4 sec-

onds each, with playback going only to

their respective channels.

The voice segments can prove useful,

allowing the facilities that receive the

vehicles signal (generally provided by

microwave or satellite links) to quickly

learn more about the origin of the trans-

mission. Information such as vehicle

identity, physical location, the names of

technical and on-air personnel, mobile

Issue 3, May 1997 Model 750 User Guide

Page 10 Studio Technologies, Inc.

Model 750

telephone numbers, and left and right

channel identifiers can easily be sent.

The voice audio source is selectable, with

the choices being either the internal micro-

phone or an externally-connected signal.

The setup section also contains a digitally-

controlled sine-wave generator which,

under software control, produces a

0-level single-tone reference and multi-

step tone sequences. The single-tone

output is selectable for either 400Hz or

1kHzthe two industry-standard frequen-

cies. The tone sequences allow frequency

response testing of a transmission chain,

with the choice of either a basic or an

advanced sequence. In the basic setting

a 1kHz 0-level reference is generated,

followed by individual 10kHz, 1kHz, and

100Hz tones. In the advanced setting a

1kHz reference is generated, followed by

14 individual tones, ranging from 15kHz to

50Hz. At the end of each sequence a left/

right channel identifier is generated. It

consists of a short burst of 700Hz sent

only to the left channel, followed by two

short bursts of 700Hz to the right channel.

While the voice and tone setup signals can

be assigned to the main and AUX output

buses, a direct line-level stereo output is

also provided. This allows the generation

of setup and test signals without interfer-

ing with normal operation of the main and

AUX audio buses.

Talk Back

The Model 750 includes a talk back

function. Two push-button switches allow

the selected voice source to be routed to

the setup direct output and/or the main

and AUX outputs.

Mounting, Connectors, and Mains

While the Model 750 is loaded with fea-

tures, it only requires two spaces in a

standard 19-inch rack. XLR-type and

¼-inch 3-conductor audio connectors are

used for audio interconnection. Mains

power is factory configured for 100, 120,

or 220/240V, 50/60Hz.

Installation

In this section you will be installing the

Model 750 Audio Mixer in an equipment

rack. Input and output connections will

be made using the Model 750s multitude

of jacks. AC mains power will then be

connected.

During normal operation, no access to the

back panel of the Model 750 is required.

However, there are five trim potentiom-

eters that may need to be adjusted prior to

the Model 750 going into service, depend-

ing on the requirements of the specific

installation. Since part of the calibration

process involves observing the front-

panel-mounted VU meters, you may want

to perform the calibration process on-the-

bench, prior to mounting the Model 750

in an equipment rack. In addition, there

are a number if DIP switches that may

need to be set. Refer to the Configuration

section of this guide for details.

Realistically, as long as you can get the

assistance of another person for about

5 minutes, go ahead and install the Model

750, wire it up, and then worry about the

calibration. You will need a highly-skilled

and articulate helper, capable of describ-

ing the position of the VU meters indica-

tors with words such as: too high,

down a little, almost, up a tad,

and OKyouve got it!

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 11

Model 750

Locating the Unit

Physical access and mechanical noise

are the primary factors when choosing a

mounting location. By its very nature as an

audio mixer, an operator will need easy

access to all the controls, as well as good

sight lines to the meters. In most cases the

internal microphone, located on the upper

right side of the front panel, will be used,

so be careful that you dont have a cooling

fan blowing directly onto it.

As a device that contains high gain audio

stages, hum and noise pickup is possible

through the chassis and associated ca-

bling. Locating the unit away from devices

that radiate strong AC fields is an excellent

idea. The left side of the Model 750, which

contains the microphone preamplifiers, is

the area most sensitive to noise pickup.

Also, it is possible that the Model 750s

power transformer may radiate energy into

adjacent equipment. It is located at the

right, bottom rear corner of the unit, and

may need to be physically isolated from

sensitive equipment, especially videotape

recorders.

Mounting

The Model 750 is intended for rack mount-

ing, requiring two standard rack spaces

(3.5 inches). It weights approximately 14

pounds (6.4kg), and operates on either

100, 120, or 200/240V, 50/60Hz. The unit is

secured to the front mounting rails of an

equipment rack using two mounting

screws per side.

Audio Inputs and Outputs

The Model 750s audio input and output

connections are made using 3-pin XLR-

type and ¼-inch 3-conductor phone jacks.

Balanced Connections

(Input & Output)

Sleeve

(Shield)

(Switchcraft No. 297, Neutrik NP3C, or equivalent)

Tip ( + )

Ring ()

Dont be concerned about our use of

phone jacks as part of a professional

product. The jacks used feature gold-

plated contacts for high reliability. As

much as we wanted to, there was simply

no room in only two rack spaces to use all

XLR-type connectors.

Caution: For reliable audio intercon-

nection, the phone plugs you use must

comply with industry-standard RS-453.

Switchcraft No. 297 or Neutrik NP3C

are highly recommended.

Mic/Line Inputs

The Model 750 contains four input chan-

nels that are compatible with microphone

or line-level sources. Each of the four

Mic/Line inputs use a single 3-pin female

XLR-type connector. Prepare the mating

connectors (males) so that pin 2 is signal

high (+ or hot), pin 3 is low ( or cold),

and pin 1 is shield.

It is expected that the mic/line inputs will

be wired to input/output connector panels

located on the side or rear of the vehicle.

Its also nice to have one or two of the

inputs multed to connectors inside the

vehicle, allowing the quick connection of

Figure 3. Recommended phone plugs

Issue 3, May 1997 Model 750 User Guide

Page 12 Studio Technologies, Inc.

Model 750

electronically balanced, have a nominal

level of +4dBu, and are capable of driving

balanced or unbalanced loads of 600

ohms or greater. Main Output 3 is trans-

former balanced, with its output level

selectable, using a front-panel button, for

microphone or line level. It is capable of

driving full level into balanced or unbal-

anced loads of 2k ohms or greater.

Main Output 1 and 2

Main Output 1 and 2 are intended for

connection to devices located within the

vehicle. It is recommended that either

Main Output 1 or 2 be used as the on-air

source. Each output utilizes two ¼-inch

3-conductor phone jacks for interconnec-

tion. Prepare the mating connectors

(plugs) so that tip is signal high (+ or hot),

ring is low ( or cold), and sleeve is shield.

Whether these outputs are wired via the

patch bay will depend on the specific

installation.

Note that while the electronically-balanced

output circuitry is capable of driving loads

of 600 ohms or greater, the output level

will drop slightly as the load impedance

approaches 600 ohms. A 0.5dB difference

in output level can be expected as the

load impedance changes from 10k ohms

to 600 ohms.

While balanced operation is preferred,

unbalanced operation is not a problem

for the electronically-balanced output

circuitry. To connect to an unbalanced

load connect the tip to high (+ or hot),

and both the ring and sleeve to shield. For

optimal unbalanced operation, it is impor-

tant to connect both the ring and sleeve

together on the plug that mates with the

Model 750.

microphone or line-level signals. To mini-

mize noise and other problems, it is rec-

ommended that the mic/line signals arent

wired by way of points on the patch bay.

Stereo Line Inputs

The Model 750 contains two stereo line-

level input channels. They are intended

for connection to sources located within

the vehicle, specifically the audio output

channels from videotape machines. The

direct-coupled, electronically-balanced

circuitry has extremely low noise, high

common-mode rejection, but no special

protection from overvoltage or static.

(Dont get us wrongthe stereo line input

circuitry is better than whats found on

most audio mixers available today

theyre just not as tough as the mic/line

inputs!) The stereo line input circuitry is

intended for connection to balanced, line-

level signals with a nominal operating level

of +4dBu.

Each stereo line input channel uses two

3-pin female XLR-type connectors. Pre-

pare the mating connectors (males) so

that pin 2 is signal high (+ or hot), pin 3 is

low ( or cold), and pin 1 is shield. With an

unbalanced source connect pin 2 to high

(+ or hot), and pins 1 and 3 to shield. If

connecting to an unbalanced source in

this manner results in hum or noise, con-

nect pin 2 to high (+ or hot) and pin 3 to

shield; leave pin 1 unterminated. For

flexibility of use it is recommended that the

stereo line input connections be routed

through points on the patch bay.

Main Outputs

The Model 750 contains three sets of

stereo outputs associated with the main

audio bus. Main Outputs 1 and 2 are

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 13

Model 750

Even if there are no immediate plans

to connect the AUX audio outputs to

external devices, it is recommended that

they be terminated on patch points. In this

manner they will be accessible for those

one-in-a-million situations that seem to

arise far too frequently!

Configurable Outputs

Each of the four configurable outputs is

electronically balanced, has a nominal

level of +4dBu, and is capable of driving

balanced or unbalanced loads of 600

ohms or greater. The outputs are intended

to be used only within the vehicle. DIP

switches are associated with each

configurable output. Details on using

these switches can be found in the Con-

figuration section of this guide.

Each of the configurable outputs is acces-

sible via a ¼-inch 3-conductor phone jack.

Prepare the mating connectors (plugs) so

that tip is signal high (+ or hot), ring is low

( or cold), and sleeve is shield. To con-

nect to an unbalanced load connect the

tip to high (+ or hot), and both the ring

and sleeve to shield.

It is highly recommended that the config-

urable outputs be routed via the patch

bay. Or, if there are no immediate needs

for these outputs, be certain to terminate

them on the patch bay for future use.

Sometime, somewhere, they will get used.

Setup Direct Output

Direct access to the setup sections voice

and tone signals is provided by means of

a stereo line-level output. As one of the

unique features of the Model 750, it should

definitely be utilized! This output is in-

tended to be connected to the audio input

associated with the video/audio switchers

Main Output 3

Main Output 3 has been optimized for

connection to devices located external

to the vehicle. This ruggedized, trans-

former-coupled stereo output is select-

able, using a front-panel button, for

microphone or line level. It is recom-

mended that Main Output 3 be wired, via

points on the patch bay, to the vehicles

input/output connector panel. Two 3-pin

male XLR-type connectors are used for

interconnection. Prepare the mating con-

nectors (females) so that pin 2 is high

(+ or hot), pin 3 is low ( or cold), and pin

1 is shield.

Main Output 3 was specifically designed

to provide a fully-isolated output with good

sonic performance. It was NOT designed

to drive full level into 600 ohm loads! While

a 600 ohm load can be connected without

damage, a 2dB level drop should be

expected. Used with 2k ohm or greater

loads, Main Output 3 will provide its full

specified performance.

AUX Output

A stereo output is provided to give access

to the Model 750s AUX audio bus. It is

intended that the AUX audio output will be

connected to devices within the vehicle.

The electronically-balanced, line-level

outputs have a nominal level of +4dBu,

and are capable of driving balanced or

unbalanced loads of 600 ohms or greater.

Two ¼-inch 3-conductor phone jacks

used for interconnection. Prepare the

mating connectors (plugs) so that tip

is signal high (+ or hot), ring is low ( or

cold), and sleeve is shield. To connect to

an unbalanced load connect the tip to

high (+ or hot), and both the ring and

sleeve to shield.

Issue 3, May 1997 Model 750 User Guide

Page 14 Studio Technologies, Inc.

Model 750

bars and tone position. For maximum

flexibility, it is recommended that these

signals be connected by way of the patch

bay. In this way they can be accessed for

test and calibration purposes.

The electronically-balanced, nominal

+4dBu outputs are accessible via two

¼-inch 3-conductor phone jacks. The

circuitry is capable of driving balanced or

unbalanced loads of 600 ohms or greater.

Prepare the mating connectors (plugs) so

that tip is signal high (+ or hot), ring is low

( or cold), and sleeve is shield. To con-

nect to an unbalanced load connect the tip

to high (+ or hot), and both the ring and

sleeve to shield.

Two trim potentiometers are used to set

the output level of the setup direct output.

Refer to the Configuration section of this

guide for details.

External Monitor Inputs

Two external audio signals can be con-

nected, allowing them to be monitored

independently of the internal audio buses.

It is intended that the external monitor

inputs will be connected to outputs associ-

ated with off-air, microwave, or satellite

receivers. Both inputs are stereo, elec-

tronically balanced, and interface using

¼-inch 3-conductor phone jacks. External

Monitor Input 1 is compatible with signals

that have a nominal level of +4dBu. Exter-

nal Monitor Input 2 is compatible with

10dBu or +4dBu signals. A DIP switch,

located on the front panel of the Model

750, is used to set the input sensitivity.

Refer to the Configuration section of this

guide for configuration details.

As an installation aid, a connection made

only to the jack labeled L/MONO routes

the signal to both the left and right input

circuitry. This allows a monaural signal to

be monitored in a 2-channel mono format.

If you wish to have a mono input con-

nected only to the left input, simply insert

an unterminated plug into the associated

right input jack. This will break the nor-

mal connections that link the left and right

input circuits.

Prepare the mating connectors (plugs)

so that tip is signal high (+ or hot), ring is

low ( or cold), and sleeve is shield. With

an unbalanced source connect the tip to

high (+ or hot), and both the ring and

sleeve to shield. If connecting to an unbal-

anced source in this manner results in

hum or noise, connect tip to high (+ or

hot) and ring to shield; leave the sleeve

unterminated.

Monitor Output

The monitor output is designed for con-

nection to an audio power amplifier asso-

ciated with a pair of monitor loudspeakers.

In mobile applications it may prove com-

mon for loudspeakers with internal amplifi-

ers, such as those from Fostex, to be

connected to the Model 750s speaker

output. The outputs are electronically-

balanced, line-level, and can drive bal-

anced or unbalanced loads of 600 ohms

or greater.

The monitor output utilizes two ¼-inch

3-conductor phone jacks for interconnec-

tion. Prepare the mating connectors

(plugs) so that tip is signal high (+ or hot),

ring is low ( or cold), and sleeve is shield.

To connect to an unbalanced load con-

nect the tip to high (+ or hot), and both

the ring and sleeve to shield. Whether

these outputs are wired via the patch bay

will depend on the specific installation.

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 15

Model 750

Record/Talk Back External

Audio Input

The Model 750 contains an internal micro-

phone which is used by the setup section

for voice recording, as well as by the talk

back functions. In special cases it may be

desirable for an external source of audio

to be used in its place. A ¼-inch 3-con-

ductor phone jack allows the connection

of a line-level signal, with a nominal level

range of 20 to +4dBu.

Prepare the mating connector (plug) so

that tip is signal high (+ or hot), ring is low

( or cold), and sleeve is shield. With an

unbalanced source connect the tip to high

(+ or hot), and both the ring and sleeve

to shield. If connecting to an unbalanced

source in this manner results in hum or

noise, connect tip to high (+ or hot)

and ring to shield; leave the sleeve

unterminated.

It is intended that a microphone associ-

ated with an intercom system headset will

serve as the external audio source. Inter-

com stations often provide an amplified,

line-level version of the mic signal. It is

often referred to as a hot microphone

output.

A trim potentiometer is associated with the

external audio input, allowing adjustment

of the input sensitivity. Refer to the Con-

figuration section of this guide for details.

Information is also provided on how to set

the front panel DIP switch to select the

external audio input.

Connecting AC Mains Power

The Model 750 is factory configured to

operate from AC mains power of either

100, 120, or 220/240V, 50/60Hz. In most

cases, units shipped to North America

are factory configured for 120V operation

and are supplied with an internal 0.5A,

5 x 20mm fast-acting fuse. Units config-

ured for 100V also utilize a 0.5A fuse. Units

configured for 220/240V operation are

supplied with a 0.25A, 5 x 20mm fast-

acting fuse.

Before connecting the Model 750 to mains

power, determine the actual mains voltage

and confirm that the Model 750 has been

configured correctly. As could be ex-

pected, an incorrect mains configuration

could seriously damage the unit.

Should it be necessary to change the

units operating voltage refer to the Ser-

vice Guide, a separate document available

upon request from Studio Technologies.

Figure 4. Detail of back panel showing voltage

configuration chart

Issue 3, May 1997 Model 750 User Guide

Page 16 Studio Technologies, Inc.

Model 750

The Model 750 uses an IEC standard

connector to mate with the AC mains cord.

Normally the cord supplied has a North-

American standard plug on one end and

an IEC connector on the other. In non-

North American applications the appropri-

ate plug must be attached. The wire colors

in the mains cord will, in most cases,

conform to the internationally recognized

CEE color code and should be wired

accordingly:

Connection Wire Color

Neutral (N) Light Blue

Live (L) Brown

Earth/Ground (E) Green/Yellow

Because the Model 750 contains no power

switch it will start operating as soon as AC

mains power is connected.

Configuration

As in life, power leads to responsibility.

The Model 750 is no exception as its

features, flexibility, and operational power

leads to a number of configuration issues

that must be reviewed once installation

has been completed. All are quite simple

and, with the exception of the front panel

configuration DIP switches, will only need

to be addressed once.

Configurable Outputs

The Model 750 contains four configurable

outputs, the likes of which arent found on

other audio mixers. Their uniqueness may

make them seem a bit confusing at first

glance. However, they arent too weird,

simply being electronically-balanced

output circuits whose input source is

selectable, rather than being fixed. The

hardest part of using the configurable

outputs is deciding which audio source, or

sources, should be assigned to which

output! By carefully reviewing the choices,

and how their selection may benefit an

installation, the power of this humble out-

put should become apparent. These four

outputs may well eliminate the need for

external distribution amplifiers, combining

amplifiers, and microphone preamplifiers.

Figure 5. Detail of back panel showing

configurable output section (typical of 4)

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 17

Model 750

The five choices are main output left, main

output right, AUX output left, AUX output

right, and mic/line direct:

 Main output left and main output right

are the same signals that connect to the

output circuitry associated with the main

outputs.

 AUX output left and AUX output right

are the same signals that connect to the

output circuitry associated with the AUX

line-level output.

 Mic/line direct is a signal that is derived

from its associated mic/line input chan-

nel. The signal is the same as that

connected to the pan-pot; post input

preamplifier, low-cut filter, and phase

reverse function. Configurable output 1

has access to the signal from mic/line

input 1; configurable output 2 has

access to the signal from mic/line input

2; etc.

Using the five DIP switches associated

with each configurable output, simply

select the source or sources desired. Note

the word sourcesmore than one

source can be selected! As an example,

selecting switch 3, AUX out left, and switch

4, AUX out right, will produce a monaural

(L+R) version of the AUX output. Selecting

switch 1, main out left, for configurable

output 1, and switch 2, main out right, for

configurable output 2 will result in another

stereo output, viola, main output 4!

It would be the authors choice to use

the configurable outputs in this way:

Configurable output 1 set for a mono

version of the main output, configurable

output 2 set for a mono version of AUX

output, and configurable outputs 3 and 4

set for mic/line direct outputs. Terminating

on the patch bay, these four outputs could

provide quite handy for those once in a

blue moon situations that seem to occur

about once a month!

The two mic/line direct outputs may prove

especially helpful, providing two channels

of mic/line amplifier for general purpose

or emergency use. In special situations, a

signal of virtually any audio level could be

bumped to nominal +4dBu.

Figure 6. Detail of back panel showing level

adjustment trim pots for setup direct output

Issue 3, May 1997 Model 750 User Guide

Page 18 Studio Technologies, Inc.

Model 750

Setup Direct Output

The setup direct output is a stereo line-

level output that provides direct access

to the voice and tone signals. Trim potenti-

ometers allow adjustment of the output

level from approximately 2 to +10dBu.

The trim pots are located on the back

panel, directly above the setup direct

outputs ¼-inch phone jacks. At the fac-

tory these outputs are loaded with 100k

ohms, and then the trim pots are adjusted

to give an output level of +4dBu.

If the setup direct output is to be used in

an installation, its important that the left

and right channel output levels be ad-

justed to precisely match the requirements

of the system. Even if you simply want to

use the standard output level of +4dBu,

its still important to confirm the factory

calibration. Also note that while the output

circuitry is capable of driving loads of 600

ohms or greater, the output level will drop

slightly as the load impedance ap-

proaches 600 ohms. A 0.5dB difference in

output level can be expected as the load

impedance changes from 20k ohms to

600 ohms.

In practice, calibrating the output level

is quite simple:

1. Confirm that both the left and right

channels of the setup direct output

are terminated with their normal load

impedance.

2. Ensure that the Model 750 is con-

nected to mains power.

3. Activate the setup sections single tone

output by placing the single tone

button to the on (in) position. The voice

and multi-tone buttons should be in

their off (out) position.

4. Using an appropriate adapter cable,

connect an audio level meter directly

across the left channel setup direct

output. Ensure that your connection

does not remove the normal load! The

level meter must be a precision device

thats intended for audio usea

general-purpose voltmeter is not

adequate!

5. Carefully observing the level meter,

adjust the trim pot associated with the

left channel to give the desired output

level.

6. Repeat steps 4 and 5, this time for the

right channel.

External Record/TB Audio

Source

During installation, an external source of

audio may have been connected to serve

in lieu of the Model 750s internal micro-

phone. This audio source will be used for

voice recording by the setup section, as

well as by the talk back functions. The

external audio source needs to have a

nominal audio level of 20 to +4dBu. A

trim potentiometer, located on the back

panel directly above the record/TB exter-

nal audio input jack, adjusts the input

sensitivity of the circuitry. For correct

operation it is crucial that this trim pot be

adjusted so that the source audio level is

matched with the requirements of the

Model 750.

The procedure is a bit convoluted, but not

difficult:

1. Ensure that mains power is connected

to the Model 750.

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 19

Model 750

2. Select the external audio source by

placing front panel configuration DIP

switch 1 to the down position.

3. Select the main bus as the VU meter

source by placing the meter source

select button to the out position.

4. Place the main output limiter/compres-

sors to the off state by placing the

button labeled LIMITER to the out

position.

5. Set the main bus master output level

controls to the unity gain position. The

front panel contains a graphic arrow

symbol and the letter U to serve as

a guide.

6. Press and hold the button labeled

TALK TO MAIN AND AUX. This con-

nects the external audio source to the

main and AUX audio buses.

7. With a normal voice signal coming

from the external audio source, adjust

the trim pot so that the VU meters

deflect to the 0 VU position on signal

peaks. You may want to enlist the help

of another person to serve as a live

audio source while you make the

adjustment. It is very important that the

external audio level reflect normal

operationif your operators or pro-

ducers love to yell and scream, set the

trim pot accordingly.

VU Meter Calibration

Two trim potentiometers allow calibration

of the VU meters normal operating level.

The range of the trim pots allow 0 on the

meters to represent an output signal level

of 2 to +10dBu. While providing a power-

ful feature, these trim pots must be ad-

justed very carefully. Be warnedmeter

calibration effects the audio performance

of the entire Model 750! From the factory,

the VU meters are calibrated so that 0 VU

represents an output level of +4dBu on

main output 1, when loaded with 100k

ohms. In certain cases it may be a good

idea to check the meter calibration, such

as when main outputs 1 and 2 are loaded

with 600 ohms, or when main output 3 is

going to serve as the primary output.

In some installations it may be desirable to

have a level different from +4dBu, such as

Figure 7. Detail of back panel showing trim

pots for VU meter calibration

Issue 3, May 1997 Model 750 User Guide

Page 20 Studio Technologies, Inc.

Model 750

0dBu, to serve as the 0 reference. While

Studio Technologies is not excited about

such a prospect, it can certainly be

achieved. There is one serious downside

to changing the nominal operating level;

the limiter/compressor threshold. The

limiter/compressors are designed to

activate at approximately 6dB above the

Model 750s internal operating level,

corresponding to an output level of

+10dBu. The threshold is not factory

calibrated and is not easily modified in the

field. If, as an example, the meters are re-

calibrated for a nominal output level of

0dBu, the limiter/compressors will now

activate at 10dB above this new nominal

level. Prior to changing the operating level,

a discussion with personnel at the factory

may be warranted. Call us for a chat!

While not difficult, meter calibration is a bit

tedious:

1. Ensure that mains power is connected

to the Model 750.

2. Place the main output limiter/compres-

sors to the off state by placing the

button labeled LIMITER to the out

position.

3. Activate the setup sections single tone

output by placing the single tone

button to the on (in) position. The voice

and multi-tone buttons should be in

their off (out) position.

4. On the setup section, place the main

bus assignment button to the on (in)

position.

5. Select the main bus as the VU meter

source by placing the meter source

button to the monitor (in) position and

selecting main stereo. This makes the

meters most accurate when they are

monitoring the main bus via the moni-

tor section. If you wish to have the

meters most accurate when they are

connected directly to the main bus, set

the meter source button to the main

(out) position.

6. Designate main output 1, 2, or, in

rare circumstances, 3 as the desired

primary output. This is the output that

the meters will be calibrated against. If

main output 3 is selected, set its output

level button to the line (in) position.

7. Confirm that the left and right channels

of the designated main output are

terminated with their normal load

impedance.

8. Using an appropriate adapter cable,

connect an audio level meter directly

across the left channel of the desig-

nated main output. Ensure that the

meter connection does not remove the

normal load. The level meter must be a

precision device thats intended for

audio usea general-purpose volt-

meter is not adequate!

9. Carefully observing the audio level

meter, set the main bus left channel

master output control so that the out-

put level matches the desired nominal

operating level. In most cases this will

be +4dBu.

10.Observing the Model 750s left channel

VU meter, adjust the left channel meter

trim pot to give 0 VU.

11.Repeat steps 8 through 10, this time

for the right channel.

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 21

Model 750

tion DIP switches. One is shipped with

each Model 750, being attached to the

front panel by means of two screws.

Where the Model 750 is physically located,

and who will be operating the unit, will

dictate whether the security cover remains

in place or is permanently removed. In a

facility where many operators have access

to the unit, it may be best to set the con-

figuration DIP switches to match the needs

of most operators, then replace the cover.

In a facility where only a few operators

have access, such as an SNG vehicle, the

cover can stay off to allow quick access to

the features.

A brief description of the function of each

configuration DIP switch is contained on

the front panel, directly above the DIP

switches. A more detailed description is

provided here:

1. REC/TB AUDIO SOURCE: EXT

▼▼

▼

/ INT

▲▲

▲

This switch selects which audio source will be

used by the voice record and talk back sections.

In the down position the external audio source is

selected. In the up position the internal micro-

phone will be used.

Be careful with this switch! In most cases an

external audio source will not be connected and

the internal microphone needs to be selected for

use. Its easy to accidentally set DIP switch 1 to

the down position and have no source of record

and talk back audio.

2. REC L+R: DISABLE

▼▼

▼

/ ENABLE

▲▲

▲

This switch selects whether the setup sections

L+R voice segment can be recorded. In the down

position recording cannot take place. In the up

position recording can take place.

This feature can prove useful where a semi-

permanent L+R voice segment needs to be re-

corded. By placing the switch to the down

position, the L+R voice segment cannot be

changed.

Configuration DIP Switches

Studio Technologies is sorry to report that

the 8 configuration DIP switches had to be

placed on the front panel. We tried to

place them on the back panel where

theyd be hard to reach, or inside the unit

where youd never find them, but we just

couldnt find the spacejust kidding! The

8 configuration DIP switches are located

on the front panel where you can easily

get at them. These switches allow impor-

tant operating parameters to be easily

configured, and then quickly changed

on-the-fly as operating requirements

dictate. The factory default is to have all

8 switches set to the up position.

A clear plastic security cover can be

used to prevent access to the configura-

Figure 8. Detail of front panel showing

configuration DIP switches

Issue 3, May 1997 Model 750 User Guide

Page 22 Studio Technologies, Inc.

Model 750

output. Also, if the VU meter source button is set

to follow the monitor section, the AUX bus signals

will automatically be connected to the VU meters

whenever AUX interrupt is active.

8. EXT 2 INPUT LEVEL: 10

▼▼

▼

/ +4

▲▲

▲

This switch is used to select the nominal input

level of external monitor input 2. In the down

position the nominal input level is 10dBu. In the

up position the nominal input level is +4dBu.

3. REC L ONLY/R ONLY: DISABLE

▼▼

▼

/ ENABLE

▲▲

▲

This switch selects whether the setup sections

L Only and R Only voice segments can be re-

corded. In the down position recording cannot

take place. In the up position recording can take

place.

This feature can prove useful where semi-

permanent L Only and R Only voice segments

need to be recorded. By placing the switch to the

down position, the L+R voice segment cannot be

changed.

4. SINGLE TONE: 400HZ

▼▼

▼

/ 1KHZ

▲▲

▲

This switch selects whether the setup sections

single tone feature will generate a 400Hz or a

1kHz sine wave signal. In the down position

400Hz will be generated. In the up position 1kHz

will be generated.

5: MULTI-TONE SEQUENCE: BASIC

▼▼

▼

ADVANCED

▲▲

▲

This switch selects whether the setup sections

multi-tone feature will generate a basic or an

advanced sequence of tones. In the down position

the basic sequence will be generated. In the up

position the advanced sequence will be gener-

ated. Refer to appendices A-1 and A-2 for detailed

descriptions of the tone sequences.

6: AUX INT SPEAKER: NO

▼▼

▼

/ YES

▲▲

▲

This switch selects whether the AUX audio bus will

automatically be selected as the audio source for

the monitor speaker whenever an input channel or

the setup section is assigned to the AUX bus. In

the down position a signal assigned to the AUX

bus will not impact the monitor speaker output. In

the up position any signal assigned to the AUX

bus will automatically be connected to the monitor

speaker output. Also, if the VU meter source

button is set to follow the monitor section, the AUX

bus signals will automatically be connected to the

VU meters whenever AUX interrupt is active.

7. AUX INT PHONES: NO

▼▼

▼

/ YES

▲▲

▲

This switch selects whether the AUX audio bus will

automatically be selected as the audio source for

the headphone output whenever an input channel

or the setup section is assigned to the AUX bus.

In the down position a signal assigned to the AUX

bus will not impact the headphone output. In the

up position any signal assigned to the AUX bus

will automatically be connected to the headphone

Figure 9. Detail of front panel showing mic/

line input channel (typical of 4)

Model 750 User Guide Issue 3, May 1997

Studio Technologies, Inc. Page 23

Model 750

Operation

While the Model 750 Audio Mixer is quite

simple to operate, there are nuances to its

design that make a detailed discussion

worthwhile. While you can certainly con-

sider the Model 750 a small audio mixer,

its roots are from the much larger audio

consoles that are used in broadcast and

recording settings. Well start with a dis-

cussion of the individual sections that

make up the Model 750. Then well review

how the sections work together to become

your audio master control.

Mic/Line Inputs

Four identical input channels are provided,

each being compatible with a microphone

or line-level signal. A detailed description

of one mic/line channel follows:

Input Sensitivity

The mic/line button is used to select the

sensitivity of the input circuitry. In the mic

(out) position, the expected nominal input

level is 55 to 35dBu. In the line (in)

position, a 40dB pad is inserted into the

circuit, making the expected nominal input

level 15 to +5dBu. The input circuitry is

compatible with a wide range of signal

levels and is protected from overload. In

most cases, an input signal whose nomi-

nal level is outside the expected range

shouldnt pose a problem.

Phantom Power

The button labeled +48V is used to acti-

vate phantom power for use with con-

denser-type microphones. The +48V

power supply and related circuitry was

carefully designed to allow virtually any

microphone that requires phantom power

to operate correctly. When the button is in

the off (out) position, power is not applied

to the mic/line input connector. When the

button is in the on (in) position, and the

mic/line button is in the mic (out) position,

+48Vdc is applied to the input connector.

When the mic/line button is set for mic,

and a condenser microphone is con-

nected, changing the phantom power

on/off status may result in loud pops

being generated. As such, it is best not

to change the position of the +48V button

while its associated input is live or

on-air.

While it is a good idea to leave the +48V

button in the off position except when

phantom power is required, no damage

should occur to dynamic or ribbon micro-

phones when theyre connected to the

input with phantom voltage present.

(Wow, thats why they call it phantom

powerif a device doesnt require power

it doesnt see it!)

Low-Cut Filter

The button associated with the bent line

graphic symbol is used to select the low-

cut filter function. (If you are a stickler for

semantics, its actually a high-pass filter

function!) In the off (out) position the filter

is not placed in the audio path, leaving the

signal essentially flat. In the on (in)

position the signal is routed through the

filter circuit, removing low-frequency

energy.

The filter is useful for eliminating wind

noise, traffic rumble, and AC mains hum. It

was carefully designed to give a steep

slope (18dB/octave), with the 3dB point

at 85Hz. For most voice applications the

filter can be left activated. For wide-band-

width signals, such as music, the filter

should be used only as necessary.

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Fostex 750 User manual

Brand: Fostex

Model: 750

Category: Musical Equipment

Type: User manual

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Fostex 750 User manual

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